A series of cats were subjected to MCAO for periods 20 minutes up to 3 hours and followed for up to 6 hours post occlusion. In addition to measuring changes in the CEI, and the rCBF, the water content of selected sites was determined at various times up to 6 hours post occlusion. The permeability of the blood-brain barrier was also assessed by Nafluoresceine tracer. Particular attention was given to acquiring a careful and accurate set of data on 20 minutes of occlusion of the MCA with the rCBF at or below 12 ml/100 gm/min. Release of the occlusion was followed by immediate and marked hyperemia, significantly higher in the caudate than in the cortex (ectosylvian gyros) even though the rCBF's were the same. The hyperemias correlated well with the permeability of the BBB to NaFl. The deleterious role of the marked hyperemias was evident also in greater edema in the caudate. Morphological study also revealed a correlation with evidence of neuron damage which tended to disappear in about 2 weeks. Longer duration ischemias, (more than 1 hr) showed greater edema but the impedance continued to return close to the preischemic values. Longer duration ischemias revealed gross and less recovery of the impedance to the preocclusion values.